Defibrillators deliver a high-voltage pulse to the heart in order to restore normal rhythm and contractile function in patients who are experiencing arrhythmia, such as ventricular fibrillation (“VF”) or ventricular tachycardia (“VT”) that is not accompanied by a palpable pulse. There are several classes of defibrillators, including manual defibrillators, implantable defibrillators, and automatic external defibrillators (AEDs). AEDs differ from manual defibrillators in that AEDs are pre-programmed to automatically analyze an electrocardiogram (ECG) rhythm to determine if defibrillation is necessary and to provide administration measures such as shock sequences and cardio pulmonary resuscitation (CPR) periods. To this end AEDs, but possibly also other types of defibrillator, comprise an ECG monitoring circuitry. Some types of defibrillators use the ECG monitoring circuitry to provide a ‘demand pacing’ function, wherein the ECG monitoring circuitry continually compares the patient's heartbeat to a desired outcome, and provides additional stimulus if the heart cannot maintain its required performance.
ECG signals are relatively weak electrical signals. The ECG monitoring circuitry needs to be sensitive enough to detect and analyze the ECG signals. The defibrillator also comprises high-voltage circuitry for generating a high-voltage pulse to be administered to the patient. The high-voltage pulse is conducted via a set of defibrillation leads to a set of defibrillation pads which are attached to the patient. Depending on the design of the defibrillator, the high-voltage circuitry and the ECG monitoring circuitry share the same pads and leads, or the ECG monitoring circuitry is connected to the patient via dedicated monitoring leads and monitoring pads.
When a high-voltage pulse is administered to the patient by means of the high-voltage circuitry, the high-voltage pulse is also transmitted to the ECG monitoring circuitry. The ECG monitoring circuitry receives substantially the full high-voltage pulse if the ECG monitoring circuitry shares the defibrillation pads and/or defibrillation leads with the high-voltage circuitry. Even if the ECG monitoring circuitry uses separate monitoring pads and monitoring leads, a significant portion of the high-voltage pulse may reach the ECG monitoring circuitry due to electrical coupling through the body of the patient. Therefore, it is usually necessary to protect the ECG monitoring circuitry in a defibrillator.
Protecting an ECG monitoring circuitry in a defibrillator is a challenging design problem. The monitoring circuitry has very high impendence and detection of improper patient connection requires detecting small changes in operating current, usually on the order of a few nanoamperes. At the same time, the monitoring circuitry will be exposed to brief high-voltage transients many times over the life of the defibrillator. Many overvoltage protection devices that work well in other applications require far too much steady-state power to be applied to such a circuit. Gas discharge tubes (GDTs), or spark gap devices, are a notable exception: they combine the attributes of moderate clamping voltages, very high impedance in their uncharged state, and fast operation. For these reasons, GDTs are the protection device of choice in ECG monitoring circuitry for modern defibrillators.
The inventors of the teachings disclosed herein have realized that gas discharge tubes (GDTs) may exhibit an unpredictable behavior with respect to their breakdown voltage and the time it takes for them to change between a high-impedance mode to a short-circuit mode. GDTs require a certain level of steady-state gas ionization activity. Without this ionization, the GDT may remain in its linear operating mode during a high-voltage pulse. The GDT may even remain in the off state for a portion of the high-voltage pulse. While in its linear operating mode the GDT only conducts a relatively small current. This means that a device to be protected by the GDT would be exposed to the high-voltage pulse for quite some time, or that a significant amount of electrical current would be discharged via the device to be protected, for example the ECG monitoring circuitry. The GDT may also take a comparatively long time (several milliseconds) to change state when first excited, risking damage to the circuit that the GDT protects.
A traditional method of promoting a prompt arc-over is to add a small amount of radioactive material to the ionizing gas in the GDT. This method is no longer desirable due to environmental and health reasons.